The present invention consists in a method of manufacturing a capacitive humidity sensor comprising the following steps, which are compatible with thin-film technology: forming a bottom electrode on an insulating substrate, depositing a humidity-sensitive polymer layer of uniform thickness on the bottom electrode leaving contact areas uncovered, activating surface bonds of the polymer layer, applying a colloidal dispersion of SiO.sub.2 or Al.sub.2 O.sub.3 particles of uniform grain size as a thin Layer to the polymer layer and subsequently drying it, depositing a cover electrode on the particles still evenly distributed on the polymer layer after the drying of the dispersion, and removing the particles together with the portions of the cover electrode overlying them.
In such capacitive humidity sensors it must be ensured that the cover electrode is sufficiently permeable to water vapor, i.e., the absorption and desorption of the water molecules must not be impeded, so that a maximum speed of response is obtained. Furthermore, no water molecules must become attached to the edges of the openings in the cover electrode. The cover electrode must have a high mechanical, electric, and chemical long-term stability, which is important if humidity measurements are performed in the presence of aggressive substances, particularly if the temperature falls below the dew point, and/or at high temperatures.
Furthermore, the capacitive humidity sensor must be insensitive to contamination, which means that the electric field of the capacitor formed by it must not pass through the cover electrode. The cover electrode must exhibit no selective sensitivity to other gases (than water vapor); in particular, it must not act catalytically on such gases. Finally, the cover electrode must be sufficiently electrically conductive.
One could try to realize these properties by using a very thin Au layer (about 5 nm) as the cover electrode, which would be electrically unstable because of its island pattern, so that it would result in capacitance changes, and could practically not be mechanically loaded; or by using porous layers of Pt, Cr or Pd, which would also prove electrically unstable, would result in high and unstable transition resistances, and, in the case of Pt and Pd, would act catalytically (e.g., on H.sub.2 and NH.sub.3); or by providing cover electrodes having a comb structure, which, if their teeth were spaced more than 1 .mu.m apart, would be sensitive to contamination because they could be penetrated by the electric field.
It is therefore the object of the invention to fulfil the above-explained requirements placed on capacitive humidity sensors better than hitherto by providing a novel method of manufacturing the same.
By the method according to the invention, the cover electrode is formed as a rugged and continuous metal layer, i.e., the aforementioned formation of islands is eliminated. In the cover electrode, evenly distributed microholes having a diameter of at least 0.1 .mu.m to 1 .mu.m, preferably 250 nm, are formed whose distribution density is so high that the mass of the cover electrode is reduced to a value at which, on the one hand, no islands are formed yet and, on the other hand, the required electric stability is reached, i.e., that despite the microholes, the cover electrode does not crack, for example. The microholes also largely avoid the above-mentioned penetration of the cover electrode by the electric field.
To form the microholes, the method according to the invention, while using conventional thin-film techniques, requires no mask, since a faithful reproduction of the image is unimportant. The even distribution of the microholes results automatically, so to speak, from the application of the SiO.sub.2 or Al.sub.2 O.sub.3 particles as a colloidal dispersion in the form of a thin, uniform layer, since the particles repel each other because of their electric charge. The particle density can be adjusted in a simple manner via the particle content of the colloid.
The polymer layer is preferably a layer of polyimide which is advantageously first applied as polyamide acid and then polymerized. In a preferred embodiment of the invention, the polyamide is activated by ion bombardment prior to the application of the colloid.
The application of the colloid to the polymer layer preferably takes place white the insulating substrate is rotating. Instead, the colloid may be sprayed on, or the insulating body is immersed in the colloid.
In another embodiment of the invention, an adhesive layer of a first metal is deposited on the particles, and the cover electrode of a second metal is deposited on this adhesive layer. The removal of the particles together with the portions of the cover electrode overlying them preferably takes place in a liquid by means of ultrasound.